In conventional cellular networks, mobile devices communicate individually with the base station and are susceptible to severe channel fading. Diversity techniques in time and frequency have been used to improve system performance but often at the expense of bandwidth efficiency. Multiple-antenna solutions have recently evolved as a further attempt to achieve spatial diversity without sacrificing system bandwidth, however, their feasibility has been largely limited by the device dimensions, especially at the mobile terminals. In view of this, user cooperation has been proposed to achieve spatial diversity by allowing different users to relay messages from the source to the destination.
Three commonly used cooperation strategies are amplify-and-forward (AF), decode-and-forward (DF), and coded cooperation (CC). CC gives an additional coding gain compared to AF and DF. With CC, the relay first decodes the received source message and then re-encodes it into a different set of parity bits before forwarding it to the destination. This is in contrast to AF and DF where the same set of parity bits is present in the forwarded message of the relay. In essence, CC utilizes a more powerful channel code and exploits spatial diversity by transmitting the parity bits through different diversity paths.
While conventional CC can achieve an additional coding gain compared to AF and DF, the improvement over its non-cooperative counterpart employing the same powerful channel code relies solely on the diversity branch provided by the cooperating user. In this regard, the coding gain of CC thus disappears if the cooperating user operates at the same receive signal-to-noise (SNR) ratio at the base station, as will be the case when users share the same mode of modulation and coding configuration and target error rate in an adaptive system with channel knowledge given at the transmitter.
Accordingly, improved systems and methods are desired for CC, which can achieve other potential benefits in addition to the spatial diversity gain. The above-described deficiencies of current designs for CC are merely intended to provide an overview of some of the problems of today's designs, and are not intended to be exhaustive. Other problems with the state of the art and corresponding benefits of CC techniques described herein may become further apparent upon review of the following description of the various non-limiting embodiments.